1. Field of the Invention
The present invention relates to a pump and metering apparatus having improved flow stability.
2. Description of Background Art
For certain analytical techniques, e.g. within medical research, access to pumps capable of low, stable flows is required. A conventional type of such a pump basically consists of a syringe and a means for actuating the syringe plunger. The latter device usually comprises a rotatably mounted screw, along which a runner engaging the screw and actuating the syringe plunger is moved by rotating the screw through either a stepping motor or a DC motor and a gear-wheel transmission. Factors which directly will influence the precision and flow stability of such a pump device are backlashes and wobbles in gear-wheel transmissions, slip-stick phenomenons of the syringe plunger, temporary absorption of energy in connections between driving motor and screw, and elongations of the pump chassis. While the flow stability achievable by such a pump device is completely sufficient for many purposes, there has recently, e.g. in biosensor technology, arisen a need for pump devices having improved flow stability performance.
While several of the above enumerated disturbing factors may be reduced or eliminated relatively easily, e.g. by replacing a DC motor and necessary transmission with a stepping motor and/or avoiding elastic couplings for eliminating slip-sticks, it was found to be more difficult to overcome the disturbances in the form of flow ripple caused by wobbles of the screw, either due to it not being completely straight or not being completely aligned with the motor axis. Such wobbles cause the runner threads to travel up and down on the thread flank of the screw, and also very small wobbles have been found to give unacceptable flow disturbances in cases where the requirements of flow stability are high.
It is known to use so-called ball screws to avoid travelling on the screw thread flank. Such ball screws are, however, relatively expensive, and the biasing of the balls must also be continuously adjusted in accordance with the wearing of the balls for them to contact the thread flank all the time.